Clutch torsion damper

ABSTRACT

A torsion damper which comprises two coaxial parts mounted for rotation relative to one another within the limits of a determined angular movement, and against the action of springs. There is also at least one intervening device which is sensitive to the angular movement between the parts and which is movable reversibly between a waiting position, in which it is inoperative, and, beyond a determined value of the movement, an operating position, in which the intervening device causes a modification of the operation characteristics of the torsion damper. The torsion damper is particularly usable in clutch plate assemblies for motor vehicles.

This is a continuation of application Ser. No. 148,274 filed May 9,1980, now abandoned.

The present invention relates generally to torsion damper devices whichcomprise at least two coaxial parts mounted for rotation relative to oneanother, within the limits of a determined angular movement, against theaction of resilient members adapted to act circumferentially between thesaid parts, these members being hereinafter referred to ascircumferential action resilient members, for at least a range of thesaid angular movement.

In practice, each of the parts in question comprises an annularcomponent, these components being parallel to one another, and theresilient members provided between them are each individually housedpartly in an aperture in the annular component of one of the said partsand partly in an aperture in the annular component of the other of thesaid parts.

As is known, a torsion damper device of this kind normally forms part ofa clutch plate assembly, particularly for a motor vehicle, in which caseone of its rotatable parts carries a friction disc intended to berotationally fixed to a first shaft, which in practice is a drivingshaft, namely the output shaft of an engine, while another of the saidrotatable parts is carried by a hub intended to be rotationally fixed toa second shaft, which in practice is a driven shaft, namely the inputshaft of a gearbox.

A device of this kind makes it possible in fact to effect regulatedtransmission of the torque applied to one of its rotatable parts whenthe other is itself subjected to a torque, that is to say to filter outthe vibrations liable to occur throughout the length of the kinematicchain, extending from the engine to the controlled wheel shafts, inwhich it is inserted.

As is also known, it is advantageous, at least for certain applicationsand in particular for application to clutch plate assemblies for motorvehicles, that, for the low values of the angular movement between thetwo rotatable parts of which a torsion damper device of this kind iscomposed, the torque transmitted between them should remain low.

This arrangement, which entails the use of circumferential actionresilient members of slight stiffness for the low torque values, in factmakes it possible in particular to eliminate gearbox noise in neutral,when the vehicle concerned is stationary (this noise being referred tohereinafter as neutral noise), particularly in the hot state.

Moreover, it appears that it is at least to a certain extent desirable,from this point of view alone, that the range of angular movement in thecourse of which these resilient members of slight stiffness thusintervene should be as wide as possible.

In practice, so that the resilient members of slight stiffness areeffectively the only members to intervene for the low torque values, theapertures in the annular component of the two coaxial parts of which thetorsion damper device in question is composed have one and the samecircumferential development from one of the parts to the other, while inthe case of the other resilient members interposed between the saidparts, which members have a higher stiffness, the apertures in theannular component of the driving part, in which apertures theseresilient members are housed, have a different circumferentialdevelopment from the corresponding apertures in the annular piece of thedriven part.

Thus, as soon as a gear speed has been engaged and the accelerator ofthe vehicle concerned is operated to achieve "traction" operation of thewhole arrangement, the resilient members of high stiffness onlyintervene after the circumferential play between the corresponding edgesof the apertures in which the members are housed, from the driving partto the driven part, has been taken up.

Likewise, when the action on the accelerator is terminated and thetorque between the two rotatable parts of which the torsion damperdevice in question is composed changes direction, the operation of thewhole arrangement then changing over to so-called "overrun", theresilient members of high stiffness, previously compressed between thedriving part and the driven part, expand until, finding a support ateach of their ends on the edges of those apertures in the driven part inwhich the members are housed for the rest configuration of the whilearrangement, which apertures are those of smaller circumferentialdevelopment, the resilient members of high stiffness again allow onlythe resilient members of slight stiffness to intervene and are thenrecompressed, and vice versa when a further action is exerted on theaccelerator.

In practice, when the action on the accelerator is terminated, thechange in direction of the torque is virtually instantaneous.

Now, whichever of the two directions is involved, the resilient membersof slight stiffness, which intervene each time for the low torquevalues, are very rapidly saturated.

The result of this, when the action on the accelerator is terminated oris exerted again, is an instantaneous and noisy rocking of one of thetwo parts of which the torsion damper disc in question is composed,relative to the other part, this rocking corresponding to the taking-upof play between the said parts because of saturation, first in onedirection and then in the other, of the resilient members of slightstiffness which intervene between the said parts.

In practice, the noise which accompanies this rocking is the louder, onthe one hand the larger the angular movement range between the tworotable parts in question, corresponding to the elastic members ofslight stiffness, and on the other hand the lower the gear ratioengaged.

Thus, two contradictory requirements have to be reconciled, one of themrelating to the use of an angular movement range in which, at low torquevalues, only circumferential action resilient members of slightstiffness intervene in order to achieve good absorption of neutralnoise, particularly for diesel engine vehicles in which the idling speedis particularly low and cyclic accelerations are particularly great, andthe other relating to the taking-up of angular play usually associatedwith such a range when changing over from "traction" operation, with theaccelerator depressed, to "overrun" operation, with the acceleratorreleased, and vice versa, this taking-up of angular play being also asource of noise and this noise being the greater, the wider the saidrange itself.

In order to overcome this difficulty, it has been proposed in Frenchpatent application No. 79/04,719, filed on the Feb. 23, 1979 andpublished under No. 2,449,828, to use a locking means sensitive tocentrifugal force against the action of return means, the locking meansbeing thus mounted for reversible movement between a retracted waitingposition, in which it is out of action and thus allows full liberty ofaction to the circumferential action resilient means interposed betweenthe two parts concerned, and an extended operating position in which,beyond a determined speed of rotation associated with the said returnmeans, it brings about positive circumferential support of one of theparts on the other for at least one of the directions of rotation, andthus puts the circumferential action resilient means out of action or,more generally, modifies the operational characteristics of the wholearrangement.

Although this arrangement has given and is still giving satisfaction,for certain vehicles it may prove defective or be difficult to use whenthe speed of rotation of the engine is low.

This is particularly the case with vehicles equipped with a dieselengine which develops a constant torque and permits the maintaining ofthe engagement of a gear speed even at low speeds of rotation.

For example, it is possible to maintain the engagement of a gear speeddown to about 1,000 revolutions per minite, while the idling speed ofthe engine is of the order of 750 revolutions per minute.

Consequently, if the driver relaxes the pressure on the acceleratorpedal at so low an engine speed, there will be only a difference of 250revolutions per minute to permit the intervention of the locking meanssensitive to centrifugal force, and this difference may be insufficientto enable this intervention to develop correctly.

The present invention relates generally to an arrangement givingsatisfaction with complete safety, even in the case of low speeds ofrotation.

More precisely, it firstly relates to a torsion damper device,particularly a clutch plate assembly, especially for a motor vehicle, ofthe type comprising at least two coaxial parts mounted for rotationrelative to one another, within the limits of a determined angularmovement, against the action of resilient members adapted to actcircumferentially between them for at least a range of the said angularmovement, the members being referred to as circumferential actionresilient members, which torsion damper device is characterised in thatit comprises at least one means, hereinafter referred to as aninterposition means, which is sensitive to the angular movement betweenthe parts, against the action of return means, and which is thus movablereversibly between a waiting position, in which it is inoperative, and,beyond a determined value of the movement, an operating position, inwhich, directly or indirectly bearing circumferentially on either one ofthe parts, it is adapted to provide circumferential support for at leastone of the resilient members, with the result that the operationalcharacteristics of the device are thereby modified.

According to a first possible embodiment of the invention, theinterposition means according to the invention is carried by one of therotatable coaxial parts of which the torsion damper device in questionis composed, and it is adapted to provide, by itself, in the operatingposition, circumferential support for at least one of the resilientmembers interposed between the parts.

As a variant, according to another possible embodiment of the invention,there are associated with the interposition means according to theinvention, on the one hand a support means, which extends axially andwith which, in the operating position, the member is adapted to bringabout positive circumferential support, and on the other hand an annularplate, which is engaged by means of recesses, such as apertures orgrooves, on the resilient members provided between the rotatable coaxialparts of which the torsion damper device in question is composed,without circumferential play at least for that member of the resilientmembers which is involved, that is to say the member with which theinterposition means is to cooperate, and with circumferential play forthe other resilient members, and which forms a piece carrying any one ofthe interposition and support means, while the other is fixed to acomponent belonging to either one of the parts, with the result that, inthis case, it is by means of the annular plate that the interpositionmeans is adapted to provide circumferential support at least for theresilient member which is involved.

Whatever the case may be, in neutral, with the vehicle stationary, theangular movement between the two rotatable parts of which the torsiondamper device in question is composed remains reduced, and theinterposition means which the device comprises according to theinvention is then inoperative; the circumferential action resilientmembers of slight stiffness and of high stiffness, which are providedbetween the parts, then all intervene freely between these parts overthe whole of the angular movement range allocated thereto, regardless ofthe width of this range.

On the other hand, when the vehicle is in motion and the angularmovement between the two rotatable parts of which the torsion damperdevice is composed becomes sufficiently large, the interposition meansaccording to the invention changes over from its abovementioned waitingposition, which is referred to in this text as the retracted waitingposition by way of convenience, to its operating position, which isreferred to in this text as the extended working position by way ofconvenience.

Thus, although the locking means according to the invention isinevitably sensitive to centrifugal force, as a component of a rotatableassembly, it is essentially in dependence on the angular movementbetween the two coaxial parts constituting the torsion damper device inquestion, and therefore in practice on the torque transmitted from oneof these parts to the other, that, for a determined value of themovement, the changeover of this locking means from its retractedwaiting position to its extended operating position is controlled in apositive manner.

In other words, this changeover is effected reliably whatever the speedof rotation of the device.

For the extended operating position of the interposition means accordingto the invention, circumferential support is made available for one ofthe resilient members, which in practice is an elastic member of highstiffness, with the result that the course of expansion of the member isinterrupted.

Everything then proceeds as if the specific geometry of the torsiondamper device in question, which geometry is associated with thecircumferential play, from the driving part to the driven part, betweenthe apertures in which the resilient members are housed, were modified,at least for one of the members.

In other words, everything proceeds as if, for the resilient member inquestion, the circumferential development of the aperture in the annularcomponent of one of the two rotatable coaxial parts in which the memberis partly housed were capable of assuming either one of two values, onevalue when the interposition means according to the invention is in theretracted waiting position and the other value when the interpositionmeans is in the extended operating position.

In this respect, the present invention further relates to a torsiondamper device, particularly a clutch plate assembly, of the typecomprising at least two coaxial parts mounted for rotation relative toone another, within the limits of a determined angular movement, andagainst the action of resilient members adapted to act circumferentiallybetween them for at least a range of the said angular movement, each ofthe parts comprising an annular component, the components being parallelto one another, and the resilient members each being individually housedpartly in an aperture in the annular component of one of the parts andpartly in an aperture in the annular component of the other of theparts, which torsion damper device is characterised in that it comprisesat least one means, hereinafter referred to as an interposition means byway of convenience, which is sensitive to the angular movement betweenthe said parts, against the action of return means, and which is thusmovable reversibly between a waiting position, in which it isinoperative, and, beyond a determined value of the movement, anoperating position, in which, for at least one of the resilient members,it either directly or indirectly takes the place of one of the radialedges of one of the apertures in which the resilient member is housed.

It is clear from the foregoing text that, depending on the position ofthe interposition means according to the invention, the operationalcharacteristics of the torsion damper device in question are different.

In fact, when the interposition means according to the invention is inthe extended operating position, the action of the resilient members ismodified and, in particular, the resilient members of slight stiffnessare prevented from intervening by themselves at low torque values.

Thus, any noise is avoided on changing over from "traction" operation to"overrun" operation, and this is the case whatever the width of theangular movement range within which the resilient members of slightstiffness intervene for the neutral position when the vehicle isstationary.

Consequently, this range can be made as wide as is necessary for goodabsorption of the neutral noise.

The characteristics and advantages of the invention will moreover beclear from the following description, which is given by way of exampleand with reference to the accompanying diagrammatical drawings, inwhich:

FIG. 1 is a partial view in elevation, with part broken away, of atorsion damper device according to the invention, the locking means withwhich it is provided being shown in the retracted waiting position;

FIG. 2 is a view in axial section of the device, taken on the sectionline II--II in FIG. 1;

FIG. 3 is another half-view in axial section of the device, taken on thesection line III--III in FIG. 1;

FIG. 4 is a view in perspective, on a larger scale, of one of theelements employed in the torsion damper device, this element being shownin isolation;

FIG. 5 repeats part of FIG. 1, the interposition means being in theextended operating position;

FIGS. 6 and 7 are graphs illustrating the mode of operation of thetorsion damper device according to the invention;

FIG. 8 is a view in axial section, taken on the section line VIII--VIIIin FIG. 10, of another torsion damper device according to the invention;

FIG. 9 is a partial view in elevation of this device, taken in thedirection of the arrow IX in FIG. 8;

FIG. 10 is a partial view in transverse section of the device, taken onthe section line X--X in FIG. 8;

FIG. 11 is a partial view in transverse section of the device, taken onthe line XI--XI in FIG. 10;

FIG. 12 is a view in axial section of a torsion spring with which thistorsion damper device is provided;

FIG. 13 is a half-view in axial section, taken on the line XIII--XIII inFIG. 14, of another torsion damper device according to the invention;

FIG. 14 is a partial view in elevation of the device, taken in thedirection of the arrow XIV in FIG. 13;

FIG. 15 is a half-view in axial section of another torsion damper deviceaccording to the invention, taken on the section line XV--XV in FIG. 16;

FIG. 16 is a partial view in transverse section of the device, taken onthe section line XVI--XVI in FIG. 15, the one or more interpositionmeans employed according to the invention being shown in the retractedwaiting position;

FIG. 17 is an isolated view in perspective of one of the locking meanswith which the device is provided;

FIG. 18 is a half-view similar to that in FIG. 16, the one or moreinterposition means being shown in the extended operating position;

FIG. 19 is a partial view, partly in elevation and partly in section, ofanother torsion damper device according to the invention, theinterposition means with which this torsion damper device is providedbeing shown in the retracted waiting position;

FIG. 20 is a view similar to that in FIG. 19, this interposition meansbeing shown in the extended operating position; and

FIGS. 21 and 22 are views respectively similar to those in FIGS. 19 and20 and relate to another modified embodiment.

In general, these figures illustrate, by way of example, the applicationof the invention to the construction of a clutch plate assembly with adamper hub.

As is known, a clutch plate assembly of this type essentially comprisesa hub 10, a hub disc 11, radially forming an annular componentsurrounding the hub 10, two annular guide discs 12, which extendparallel to the hub disc 11, on either side of the latter, around thehub 10, and which are connected to one another by means of short axialcolumns 13 passing through the hub disc 11 by means of grooves 14provided for this purpose at the periphery of the latter, and a frictiondisc 15, which carries friction linings 16 at its periphery and oneither side of the latter.

In the illustrative embodiment shown in FIGS. 1 to 5, the friction disc15 is fixed to the guide discs 12 whilst being joined to one of thelatter by means of the same columns 13 which fix one of these guidediscs 12 to the other, the guide discs 12 are free to rotate about thehub 10 within the limits of a determined angular movement, as willbecome apparent below, and the hub disc 11 is fixed to the hub 10.

A clutch plate assembly of this type thus comprises two coaxial partsmounted for rotation relative to one another, within the limits of thedetermined angular movement mentioned above, namely a part A, which isformed by the friction disc 15 and the guide discs 12 and is referred toin this text as the exciter part by wa of convenience, this part A ingeneral being intended to be integral in rotation with a driving shaft,which is the output shaft of the engine in the case of a motor vehicle,and a part B, which is formed by the hub disc 11 and the hub 10 and isreferred to in this text as the excited part by way of convenience, thispart B in general being intended to be integral in rotation with adriven shaft, which is the input shaft of a gearbox, for example, in thecase of a motor vehicle.

Parallel to one another, these parts A, B comprise at least one annularcomponent, namely the hub disc 11 in the case of the part A and one orother of the guide discs 12 in the case of the part B, as in FIG. 2.

The angular movement between the parts A, B constructed in this waytakes place against the action of resilient members 18A, 18B, which areeach adapted to act circumferentially between them for at least a rangeof the said angular movement, and which are each individually housedpartly in an aperture 19 in the hub disc 11 and partly in apertures 20in the guide discs 12.

In the illustrative embodiment shown, these resilient members arehelical springs which essentially extend tangentially relative to acircumference of the whole arrangement.

For the spring 18A, which is a spring of relatively slight stiffness andwhich is the only such spring in the illustrative embodiment shown, thecircumferential development of the aperture 19 in the hub disc 11 inwhich the spring is housed is equal to the circumferential developmentof the apertures 20 in the guide discs 12 in which the spring is alsohoused.

On the other hand, for the springs 18B, which are springs of relativelyhigh stiffness and of which there are four in the illustrativeembodiment shown, being arranged in group of two on either side of thespring 18A, the circumferential development of the apertures 19 in thehub disc 11 is greater than the circumferential development of theapertures 20 in the guide discs 12, with the result that, between thecorresponding radial edges of these apertures, there is acircumferential play J1 for that direction of rotation of the clutchplate assembly which, as shown by an arrow F1 in FIG. 1, corresponds to"traction" operaton of the latter, and a circumferential play J2 for thereverse direction of rotation, which corresponds to "overrun" operationof this clutch plate assembly.

For example, and as shown, the circumferential play J1, measuredangularly, can be less than the circumferential play J2.

In the illustrative embodiment shown, the plays J1, J2 are identical forall the springs 18B of high stiffness, but it is self-evident that thesituation can be otherwise.

In the illustrative embodiment shown, the clutch plate assembly inquestion comprises two friction washers 21, which are each arrangedrespectively on either side of the hub disc 11, in contact with thelatter, and a corrugated washer 23, with axial elasticity, which bearson a guide disc 12 and acts on the corresponding friction washer 21 viaa distribution washer 24 integral in rotation with this guide disc 12.

A clutch plate assembly of this type is in itself well known; it willnot be described in greater detail in this text.

Its operation is also well known.

It is illustrated by the graph in FIG. 6, in which the angular movementD between the parts A, B is plotted on the abscissa and the torque Ctransmitted from one of these parts to the other is plotted on theordinate.

When a driving torque is applied to the exciter part A in the directionof the arrow F1 in FIG. 1, it is progressively transmitted to theexcited part B by means of the springs 18A, 18B.

In a first stage, for low torque values, this being the case in neutralwith the vehicle stationary, only the spring 18A of slight stiffnessintervenes in order to eliminate the corresponding neutral noise, thesprings 18B of high stiffness being at a distance from the correspondingedge of the apertures 19 in the hub disc 11 in which the said springsare housed, because of the circumferential play J1 specified above (linea on the graph).

In a second stage, as soon as the couple increases for "traction"operation, the spring 18A of slight stiffness is saturated, and thecircumferential play J1 is taken up, for a value D1 of the angularmovement corresponding to this circumferential play J1.

The springs 18B of high stiffness then come into action until they aresaturated, this saturation occurring for a value D2 of the angularmovement forming the corresponding limit of the latter (line a+b on thegraph).

For "overrun" operation, a similar procedure develops and the springs18A, 18B, which were previously compressed, first expand until thecircumferential play J1 for the springs 18B of high stiffness reappears,and then, as soon as the torque cancels out and becomes negative, thisbeing virtually instantaneous, the springs are recompressed, the spring18A of slight stiffness being the first to be recompressed and thesprings 18B of high stiffness only intervening for an angular movementD'1 corresponding to the taking-up of the circumferential play J2specified above.

Overall, the spring 18A of slight stiffness thus acts by itself in aneutral noise filtration zone P extending from the angular movement D1,for "traction" operation, to an angular movement D'1, for "overrun"operation, and the springs 18B of high stiffness only add their effectto the spring 18A of low stiffness on either side of this neutral noisefiltration zone P.

Of course, in order to simplify the graph in FIG. 6, no account has beentaken, in this text, of the hysteresis phenomenon which conjointlydevelops because of the internal friction of the clutch plate assembly.

According to the invention, an interposition means 26 is incorporatedinto this clutch plate assembly with a damper hub, which clutch plateassembly thus comprises two coaxial parts A, B mounted for rotationrelative to one another, within the limits of a determined angularmovement, and against the action of resilient members 18A, 18B adaptedto act circumferentially between them for at least a range of the saidangular movement, and the said interposition means is sensitive to theangular movement between the said parts, against the action of returnmeans, and is thus movable reversibly between a retracted waitingposition, in which it is inoperative, and, beyond a determined value ofthe said movement, an operating position, in which, bearingcircumferentially on either one of the said parts A, B, either directlyor indirectly, it is adapted to provide circumferential support for atleast one of the said elastic members 18A, 18B, with the result that theoperational characteristics of the whole arrangement are modified.

In the illustrative embodiment shown in FIGS. 1 to 5, only oneinterposition means 26 is provided.

This interposition means 26 is carried by the excited part B and it isadapted to provide, by itself, circumferential support for one of thesprings 18B of high stiffness, as will become apparent below.

In practice, in the illustrative embodiment shown in FIGS. 1 to 5, theinterposition means 26 is formed by two identical brackets which areeach arranged respectively on either side of the hub disc 11, betweenthe latter and the corresponding guide disc 12, and are conjointlymounted for rotation on this hub disc 11, about a pin 27 parallel to theaxis of the whole arrangement; by means of the pin 27, the interpositionmeans constructed in this way thus bears circumferentially on the hubdisc 11.

At one of its ends, the brackets of which the interposition means iscomposed conjointly form a beak 28 by means of which, in the operatingposition, as in FIG. 5, the interposition means is capable of hookingcircumferentially onto the particular spring 18B of high stiffness.

At its other end, the brackets of which the interposition means iscomposed each possess a lug 29 at right angles, in the oppositedirection to the hub disc 11, which lug passes through the correspondingguide disc 12, with play, by means of an elongate hole 30 provided inthe latter for this purpose, and engages, on the other side of thisguide disc 12, with a spring 31 carried by the latter.

The two springs 31 used in this way conjointly form the return meansassociated with the interposition means 26.

Each of these springs 31 comprises a central part 32, by means of whichit is engaged on the corresponding guide disc 12, at the periphery ofthe latter, by means of two grooves 33, at right angles, which thelatter possesses for this purpose, and two lateral flanges 34, whichextend approximately radially along this guide disc 12, in the directionof the axis of the whole arrangement, and are resiliently engaged on thecorresponding lug 29 of the interposition means 26.

For relatively low values of the angular movement between the parts A,B, and hence for relatively low values of the torque transmitted by thesaid parts, the interposition means 26 remains in the retracted waitingposition, as in FIG. 1, in which its retaining nose 28 is at a distancefrom the particular spring 18B of high stiffness.

The operation of the clutch plate assembly in question is then asdescribed above, and the graph representing this operation is as in FIG.6.

For a sufficient, determined, value of the angular movement between theparts A, B, and hence of the torque transmitted by the latter, theinterposition means 26, under the effects of the springs 31 which arecarried along by the guide discs 12, changes over to the extendedoperating position, as in FIG. 5.

Consequently, if the operation of the whole arrangement changes overfrom a "traction" operating mode to an "overrun" operating mode, theretaining nose 28 of the interposition means 26 is interposed on thepath followed by the corresponding end of the particular spring 18B ofhigh stiffness in the course of its expansion which is characteristic ofthis changeover, with the result that the course of expansion of thisspring 18B of high stiffness is therefore interrupted.

For the purpose of simplification, it will be supposed hereafter, on theone hand that this interruption of the course of expansion of theparticular spring 18B of high stiffness intervenes for the angularmovement D1 specified above, and that, at rest, this spring 18B of highstiffness has a prestress which is greater than the stiffness of thespring 18A of slight stiffness.

This spring 18B of high stiffness, which is then supported on the onehand, by one of its ends, on the nose 28 of the interposition means 26according to the invention, which is linked to the hub disc 11, and onthe other hand, at its other end, on the guide discs 12 and the hub disc11, and which is thus acted upon in compression, momentarily opposes theexpansion of the spring 18A of slight stiffness.

This results in a modification of the operational characteristics of theclutch plate assembly, compared with the previous characteristics, theinterposition means 26 according to the invention having taken theplace, for the spring 18B in question, of the corresponding radial edgeof the aperture 19 in the hub disc 11 in which the spring is housed, andeverything proceeding as if the circumferential development of thisaperture were modified in this way.

In practice, as shown on the graph in FIG. 7, this modification resultsin a sudden drop in the torque for the angular movement D1, and then inthe continuation of the decrease in this torque and in the angularmovement with a gradient which is greater than that resulting from thestiffness of the spring 18A of slight stiffness alone.

Thus, this spring 18A of slight stiffness is prevented from interveningalone for low torque values if the torque has exceeded a determinedvalue beforehand, and this avoids noisy rocking of this clutch plateassembly during the changeover from direct operation to overrunoperation.

Of course, for low torques, as soon as the angular movement between theexciter part A and the excited part B again becomes less than adetermined value, corresponding to the value of the abscissa point D1 inthe illustrative embodiment envisaged, the interposition means 26returns to the retracted waiting position under the action of thesprings 31 which constitute the return means thereof.

However, friction means, which are not shown in the figures, arepreferably provided in order to delay this return of the interpositionmeans to the retracted waiting position.

If the spring 18B of high stiffness, with which the interposition means26 cooperates, has no prestress at rest, the drop in torque for theangular movement D1 does not take place along an approximately verticalline, as previously, but along a slightly inclined line.

Of course, several springs 18B of high stiffness can be involved in theintervention of one or more interposition means 26, and, in practice, atleast two interposition means 26 are provided, in positionssymmetrically opposite one another, in order to prevent theirintervention from inducing any radial component along the axis of thewhole arrangement.

However, at least for certain applications, it can be advantageous forthe number of springs 18B involved in the intervention of one or moreinterposition means according to the invention to be such that it doesresult in a radial component along the axis of the whole arrangement.

In fact, in this case, a variable radial hysteresis develops, the radialdisplacement of the part A relative to the part B, due to this radialcomponent, inducing radial friction either between the guide discs 12and the friction washers 21 or between the friction washers 21 and thehub disc 11.

For other applications, it can likewise be advantageous for all thesprings 18B in question to be involved in the intervention of one ormore interposition means according to the invention; in fact, thisresults in the production of a particular hysteresis, the said springsacting conjointly as a single unit, at constant torque, for a portion ofthe angular movement between the parts A, B in question.

In the modified embodiments illustrated by FIGS. 8 to 13, there areassociated with the interposition means 36 employed according to theinvention, on the one hand a support means 37, which extends axially andwith which, in the extended operating position, the interposition meansis adapted to bring about positive circumferential support, and on theother hand an annular plate 38, which is engaged by means of recesses 39on the resilient members 18A, 18B provided between the parts A, B of theclutch plate assembly in question, without circumferential play for atleast one of the members, and with circumferential play for the others,as will become apparent below, and which forms a component carryingeither one of the means comprising the interposition means 36 and thesupport means 37, whilst the other of the said means is fixed to acomponent belonging to either one of the said parts A, B.

In practice, these FIGS. 8 to 11 illustrate, by way of example, theapplication of the invention to a clutch plate assembly of the typedescribed in the French Patent filed on Apr. 24, 1974 under No.74/14,147 and published under No. 2,268,994, and also in the Addition,attached to this patent, filed on Aug. 20, 1974 under No. 74/28,507 andpublished under No. 2,282,577.

This clutch plate assembly is similar to the one described above, buthas the following differences: firstly, two springs 18A of slightstiffness are provided in diametrically opposite positions and aremounted in opposition, one of these springs being compressed while theother expands, and vice versa, depending on the direction of rotation;secondly, the springs of high stiffness are divided into two groups 18B,18B', the circumferential plays associated with the springs 18B beingrespectively greater than the circumferential plays associated with thesprings 18B', with the result that the intervention of the springs 18B'is delayed relative to that of the springs 18B, both in one direction ofrotation and in the other (not shown in detail in the figures).

In these embodiments, there are two interposition means 36, arranged indiametrically opposite positions; a support means 37 is associated witheach of these interposition means, and the interposition means 36 arecarried by the part B, and more precisely by the hub 10, the associatedsupport means 37 themselves being carried by the annular plate 38.

However, it is self-evident that a reverse arrangement could be adopted,the interposition means 36 being carried by the annular plate 38 and thesupport means 37 being carried by the part A.

It is also self-evident that the part B could take the place of the partA in either case.

In practice, in the embodiments shown, the annular plate 38 associatedwith the interposition means 36 is inserted axially between the hub disc11 and the friction disc 15, at right angles to the radial part of thebearing 47 which is normally inserted between this friction disc 13 andthe corresponding guide disc 12, on the one hand, and the hub 10, on theother hand, and the recesses 39 which the annular plate possesses forits engagement on the springs 18A, 18B, 18B' consist of grooves whichthe pate possesses at its internal periphery.

In the illustrative embodiment shown, it is on the springs 18B that thisannular plate 38 is engaged, without play, by means of its correspondingrecesses 39.

In other words, for the springs 18B, the recesses 39 are matched to thecircumferential development of these springs, for the desired engagementwithout play, the said circumferential development corresponding to thatdefined by the corresponding apertures 20 in the guide discs 12.

On the other hand, for the springs 18A and 18B', the recesses 39 in theannular plate 38 extend beyond the said springs circumferentially oneither side.

Likewise, for the passage of the axial columns 13, this annular plate 38possesses elongate holes 49 through which the said columns 13 pass withplay.

Thus, in such an embodiment, the annular plate 38 is mounted so as tofloat, this annular plate 38 being in contact, radially, in its plane,only with the springs 18B, by means of the radial edges of itscorresponding recesses 39.

This arrangement advantageously makes it possible to compensate possiblefactory tolerances and/or assembly tolerances.

However, it is self-evident that, if desired, rigid centering of theannular plate 38 can be achieved either by means of the axial columns 13or by means of the bearing 47, through contact with these columns orthis bearing.

According to the embodiments shown, the interposition means 36 employedaccording to the invention consist of simple plates.

For the holding and guiding of these plates, the hub 10 of the part Aradially carries a flange 40, which is integral therewith in rotation,this flange being, for example according to a technique which is initself known, engaged by force on the hub 10 and crimped onto thelatter, on the outside of the volume formed by the guide discs 12, onone or other side of the clutch plate assembly.

This flange 40 comprises two guides which extend radially and with eachof which an interposition means 36 is engaged.

In the illustrative embodiment shown, this guide simply consists of arecess 41 in the flange 40, and the corresponding interposition means 36is simply mounted for sliding in this recess 41.

For keeping an interposition means 36 axial, two strips 42 are joined tothe latter, for example by welding, on either side of the bracket 40,and these strips 42 extend circumferentially beyond the correspondingrecess 41, as in FIGS. 8 and 9.

At the end of each recess 41, the flange 40 possesses a retaining nose44, which circumferentially protrudes into the said recess 41, as inFIG. 9.

Conjointly, the support means 37 associated with the interposition means36 consist of lugs which extend axially in the direction of the flange40, by means of passages 45 in the friction disc 15, and which extendaxially to a sufficient degree to interfere with the path of theinterposition means 36 in the recesses 41 in which the plates aremounted for sliding.

These lugs 37 are carried by the annular plate 38.

Beyond a shoulder 50 capable of cooperating in abutment with thecorresponding retaining nose 44, each plate 36 comprises a radialextension 51 capable of being circumferentially gripped between thisretaining nose 44 and the corresponding axial lug 37.

In practice, for the rest position shown in FIG. 9, the circumferentialdevelopment L between, on the one hand, the lateral edge 52 of an axiallug 37 which is circumferentially closest to the corresponding retainingnose 44, and, on the other hand, the lateral edge 53 of thecorresponding interposition means 36 which is circumferentially furthestfrom this retaining nose 44. at most corresponds, angularly, to thecircumferential play J1 defined above.

Moreover, of course, each retaining nose 44 has a sufficiently reducedcircumferential development for it not to oppose an engagement, oppositeits own lateral edge, of the radial extension 51 of the correspondingplate 36.

In the illustrative embodiment shown in FIGS. 8 to 11, the twointerposition means 36 share the return means which are associatedtherewith, and the said means consist of a torsion spring 56, of whichthe central torsion part 57 surrounds the hub 10, and of which the arms58 are each respectively engaged with the interposition means 36 bymeans of an axial return 59, this return 59 being engaged in a passageprovided for this purpose in this interposition means and being bentback at its end beyond this passage.

In the illustrative embodiment shown, the torsion spring 56 is axiallyarranged between the flange 40 and the corresponding guide disc 12, butit is self-evident that, as a variant, it can be arranged axially beyondthis flange 40, relative to this guide disc 12; this is moreover thecase in the modified embodiment illustrated in FIGS. 13, 14.

According to a characteristic of the invention, and as shown in FIG. 12,the central torsion part 57 of the spring 56, from which part the arms58 of the spring originate, results from the winding together, in acoil, of the two strands of a wire bent into a U beforehand.

In other words, this central torsion part 57 possesses an end, it formsan elbow 60 at this end, and the transverse plane, perpendicular to theaxis of the whole arrangement, which passes through this elbow 60 is aplane of symmetry S for the spring 56; this plane of symmetry S isrepresented diagrammatically by a broken line in FIG. 12.

At the point where it is connected to the central part 57 from which itoriginates, each arm 58 to the spring 56 forms an elbow 61 in thedirection of the corresponding interposition means 36, as in FIGS. 9 and11.

At the internal periphery of the central torsion part 57 of the spring56, plates at right angles are joined at various places, for example bywelding, to the guide disc 12 in question, in order suitably to holdthis torsion spring 57 (not shown in detail in the figures).

Moreover, for changeover of the plates 36 to the extended operatingposition, the central torsion part 57 of the torsion spring 56 isangularly locked on the hub 10 which it surrounds and which belongs tothe excited part B, and a shoulder 63 is associated with each of thearms 58 of this torsion spring 56, which shoulder is fixed to theexciter part A and on which this arm 58 is capable of bearing by meansof its elbow 61.

In the illustrative embodiment shown, the central torsion part 57 of thetorsion spring 56 is fixed to the hub 10 by gripping, this central partbeing arranged between, on the one hand, two diametrically oppositebosses 65 on the flange 40 fixed to the hub 10, and, on the other hand,lugs 66 joined to these bosses 65, for example by welding; in thefigures, only one of these bosses 65 is shown, as in FIG. 8.

Moreover, in the illustrative embodiment shown, each shoulder 63 isformed at the end of a lug 67 which is joined, for example by welding,to the corresponding guide disc 12 which belongs to the exciter part A,as in FIG. 11.

When the vehicle is stationary, as in FIG. 9, the plates of which theinterposition means 36 according to the invention are composed occupy,in practice, their retracted waiting position, bearing against thebottom of the corresponding recess 41 in the flange 40.

These of the interposition means 36 are then inoperative.

If, for traction operation with the clutch plate assembly rotating inthe direction of the arrow F1 in FIG. 9, a torque is applied to thefriction disc 15 in the direction of this arrow F1, only the springs 18Aof slight stiffness initially intervene, between the two parts A, B ofwhich the clutch plate assembly is composed, for the transmission ofthis torque; subsequently, first the springs 18B and finally the springs18B' come into action successively.

However, for this traction operation and in proportion with thecorresponding angular movement between the exciter part A and theexcited part B, the shoulders 63 provided for this purpose of theexciter part A push the elbows 61 of the arms 58 of the torsion spring56 and, under this action, these arms gradually cause the radialdisplacement of the interposition means 36 along their guides, in thedirection of the corresponding retaining nose 44.

Conjointly, the axial lugs 37 on the annular plate 38 progressively moveaway from these retaining noses 44 circumferentially, the annular plate38 then being linked in rotation to the guide discs 12 by means of thesprings 18B because, by means of its corresponding recesses 39, thisplate is engaged on these springs without play.

For a determined value of the angular movement between the exciter partA and the excited part B, which can correspond, for example, to thevalue of this movement for which the springs 18B come into action, theplates 36, under the action of the arms 58 of the spring 56, occupy anextended operating position in which they are radially in abutment, viatheir shoulder 50, against the retaining noses 44, and their radialextension 51 is circumferentially inserted between these retaining beaksand the axial lugs 37 on the annular plate 38.

If operation continues to be "traction" operation, that is to say if thetorque to be transmitted between the parts A, B of which the clutchplate assembly in question is composed continues to increase, theangular movement between the said parts A, B continues to increase up toits final value, following a procedure similar to that described above.

On the other hand, if, by relaxing the depression action previouslyexerted on the accelerator of the vehicle in question, the operation ofthe whole arrangement changes over from this "traction" operation to"overrun" operation, and if the excited part B of the clutch plateassembly, which consists of the hub 10 and the hub disc 11, consequentlyundergoes a relative angular movement which is retrograde relative tothe exciter part A of this clutch plate assembly, which consists of theguide discs 12 and the friction disc 15, the plates forming theinterposition means 36 according to the invention are then capable ofintervening in the course of expansion of the springs 18B of highstiffness, during this changeover from a "direct" operating mode to an"overrun" operating mode, providing positive circumferential support forthese springs 18B via the annular plates 38.

In fact, because they bear circumferentially on the excited part B,since they are integral therewith in rotation, and because their radialextension brings about positive circumferential support with the axiallugs 37 of the support plate 38, firstly as a result of their extendedoperating position and secondly as a result of the retrograde angularmovement of the excited part B to which they are linked, they thenoffer, at the end of this retrograde movement, positive circumferentialsupport for the support plate 38.

The operational characteristics of the clutch plate assembly in questionare thereby modified, the interposition means 36 thus taking the place,via the annular plate 38, of the corresponding radial edges of theapertures 19 in the hub disc 11, in which apertures the springs 18B ofhigh stiffness are housed, and therefore modifying the circumferentialdevelopment of these apertures.

In practice, any filtration range due only to the springs 18A of slightstiffness is then eliminated.

For the extended operating position of the interposition means 36, playcan remain circumferentially between the radial extension 51 of theseplates 36 and the corresponding retaining nose 44, the bearing of theseplates 36 on the flange 40 taking place, by means of their correspondinglateral edge, in the corresponding recess 41 in this flange 40; as avariant, the radial extension 51 of an interposition means 36 can bearcircumferentially on the corresponding retaining nose 44, the retainingbeaks 44 then serving the dual purpose of circumferential support andradial retention.

Of course, as previously, for low torques, as soon as the angularmovement between the exciter part A and the excited part B drops again,the interposition means 36 escape from the axial lugs 37 on the supportplate 38 and are progressively returned to the retracted waitingposition by means of the arms 58 of the torsion spring 56 to which theyare coupled, and, also as previously, friction means, which are notshown in the figures, are preferably provided between theseinterposition means 36 and the recesses 41 in which the plates aremounted for sliding, in order to delay the return to the retractedwaiting position.

In the foregoing text, the elastic members involved with theinterposition means according to the invention are only the springs 18Bof high stiffness, the circumferential development L specified abovehaving a value appropriate for this purpose.

However, it is self-evident that, depending on the desired effects, thisvalue L can be modified, for example so that the springs 18A of slightstiffness are also involved, or so that the springs 18B' of highstiffness are the only ones involved, the annular plate 38 consequentlybeing installed.

In general, whatever the embodiment of the invention, an interpositionmeans according to the invention can intervene on any one of the elasticmembers circumferentially interposed between the parts A, B.

According to the modified embodiment illustrated in FIGS. 13 and 14, cammeans are provided between the interposition means 36 and the annularplate 38.

In the embodiment shown, these cam means comprise, for eachinterposition means 36, a finger 70, axially carried by an angle-piece71 fixed to the corresponding axial lug 37 of the annular plate 38, anda slot 72 which has an oblique flank 73 and is provided in thisinterposition means 36, the finger 70 being engaged in the said slot 72and bearing against the oblique flank 73 of the latter.

In practice, in the illustrative embodiment shown, the slot 72 possessesa circumferential extension 74 at the end of its oblique flank 73.

The operation of this modified embodiment is similar to that describedabove; in proportion with the angular movement between the exciter partA and the excited part B, the fingers 70 associated with theinterposition means 36 progressively bring about the radial displacementof the latter, which, for a determined value of this movement, causesthem to be inserted circumferentially, by means of their radialextension 51, between the retaining nose 44 and the axial lugs 37.

If this angular movement continues, the fingers 70 follow thecircumferential extensions 74 of the slots 72 without interfering withthe operation of the whole arrangement.

In the modified embodiment illustrated in FIGS. 15 to 18, theinterposition means employed according to the invention are levers 76,which are each mounted for rotation on the hub disc 11 by means of apivot 77, and which are each arranged in a recess 78 in the annularplate 38, which recess is provided for this purpose at the internalperiphery of the latter.

In the illustrative embodiment shown, the support means associated withthis lever 76 is an abutment shoulder 80, which is provided for thispurpose on the annular plate 38, and which, formed in this illustrativeembodiment by means of the corresponding recess 78 in the said plate,extends axially as above.

Moreover, the return means associated with this lever 76 comprise aspring 81, which, at one end, is coupled to this lever 76, and which, byits other end, is coupled to the annular plate 38 by means of the otherrecess 78 in the latter.

For the changeover of a lever 76 from its retracted waiting position toits extended operating position, cam means are provided between thislever 76 and the annular plate 38.

In the illustrative embodiment shown, these cam means comprise a ramp82, which is provided on the innermost circumferential edge of thecorresponding recess 78 in the annular plate 38, and a ramp 83, which isprovided on the corresponding edge of the lever 76 in question.

During traction operation, the exciter part A is caused to rotate in thedirection of the arrow F3 in FIG. 16 and it rotates in the samedirection relative to the excited part B, whilst at the same timecarrying the latter along, and, in a first stage, for a first range ofthe corresponding angular movement, only the springs 18A of slightstiffness intervene.

For this first range of the angular movement, the levers 76 remain inthe retracted waiting position, as in FIG. 16, under the action of thesprings 81.

This situation continues until the torque transmitted from the exciterpart A to the excited part B increases and the angular movement betweenthese parts A, B reaches a determined value, as above.

Consequently, under the control of the ramps 82 on the annular plate 38,the levers 76 change over to the extended operating position, as in FIG.18, in which the free end of each of these levers lies approximately onone and the same circumference as the abutment shoulder 80 provided forthis purpose on this annular plate 38.

If a reversal of torque then takes place, which is characteristic of achangeover from traction operation to overrun operation, the buttresslevers 76 engage with the annular plate 38, the consequences being thesame as above.

As soon as the torque has returned to a sufficient value, the levers 76are returned to the retracted waiting position by means of the springs81.

Of course, as above, friction means are provided between the pivot 77 ofthis lever 76 and the housing of the hub disc 11 in which this pivot isengaged, in order to delay the return of this lever to the retractedposition.

According to the modified embodiments illustrated in FIGS. 19 to 22, inwhich the exciter part A and excited part B are only shown in asimplified manner, the interposition means employed forms a bolt 102,which is mounted for radial sliding in a guide 103 fixed to a first ofthese parts A, B, opposite a ring 104 fixed to the second of the parts,the ring possessing an opening 105, forming a keeper for the bolt 102,and being carried by a hub 106 which is provided on the surface by cammeans 107 capable of acting upon the bolt 102 in the direction of thekeeper 105, these cam means 107 being angularly arranged at right anglesto the opening forming the keeper 105.

In the embodiment illustrated in FIGS. 19 and 20, the guide 103 belongsto the exciter part A, while the ring 104 belongs to the excited part B.

As a variant, as in FIGS. 21 and 23, the guide 103 belongs to theexcited part B, while the ring 104 belongs to the exciter part A.

Whatever the case may be, when the vehicle is stationary, as in FIGS. 19and 21, the guide 103 and hence the bolt 102 are angularly at a distancefrom the opening forming the keeper 105, and from the cam means 107.

On the other hand, as soon as the angular movement of the exciter part Aand the excited part B reaches a determined value, for example the valueD1 specified above, the bolt 102 is located opposite the opening formingthe keeper 105, and, under the thrust of the cam means 107, which inpractice are a simple ramp, the bolt penetrates into this opening, as inFIG. 20.

The exciter part A therefore finds positive circumferential support onthe excited part B, as above, through the guide 103, the bolt 102, thering 104 and the hub 106.

Of course, return means are provided in order to return the bolt 102 toits retracted rest position when the angular movement between theexciter part A and the excited part B again becomes less than the valueD1.

Moreover, as above, friction means are preferably provided between thebolt 102 and the guide 103 in order to delay the return of this bolt tothe retracted rest position.

As will have been understood, in all the embodiments described above,the interposition means employed according to the invention is or areinevitably sensitive to centrifugal force, and the latter can have apositive effect on these means, in practice at sufficiently high speeds.

However, the changeover of this or these means from their retractedwaiting position to their extended operating position is effected in apositive manner, independently of this speed.

As is the case in practice, this changeover can thus take place atinsufficiently high speeds for the centrifugal force to have anysubstantial effect.

It is easy to understand from the foregoing text tha the invention iscapable of being put into effect in a wide diversity of applications.

Thus, it is not limited to the embodiments and forms of applicationwhich have been more particularly described and shown, but encompassesany variant.

In particular, it can equally well be applied to the torsion damperdevice comprising three coaxial parts mounted for rotation relative toone another, in pairs, as is the case, for example, in the clutch plateassembly described in detail in the French Patent filed on Aug. 29, 1973under No. 73/31,172 and published under No. 2,242,606, and also in theAddition, attached to this patent, filed on Apr. 12, 1974 under No.74/12,915 and published under No. 2,270,491.

In this clutch plate assembly, the interposition means according to theinvention can intervene between any one or other of the variousrotatable coaxial parts which it comprises.

Moreover, instead of intervening during the changeover from tractionoperation to overrun operation, the interposition means according to theinvention can equally well intervene during the changeover from overrunoperation to traction operation.

Finally, the field of application of the invention is obviously notlimited only to that of clutch plate assemblies for motor vehicles, butcovers the entire field of application of torsion damper devices.

I claim:
 1. A torsion damper assembly of the type including at least twocoaxial parts mounted for relative angular movement within predeterminedlimits of angular displacement, circumferentially acting resilientmembers opposing said relative angular movement of said coaxial partsover at least a range of angular displacement, the improvementcomprising at least one intervening means responsive to relative angularmovement between said coaxial parts for movement between a standbyposition and an operative position beyond a predetermined angulardisplacement, return means for urging said intervening means from saidoperative position toward said standby position, said intervening meanseffectively circumferentially bearing against one of said coaxial partsand engageable with one of said resilient members to define acircumferential abutment for said one resilient member in said operativeposition for modifying the operational characteristics of said torsiondamper assembly.
 2. The torsion damper assembly of claim 1, wherein saidintervening means is carried by said one of said coaxial parts anddefines a circumferential support for said one resilient member.
 3. Thetorsion damper assembly of claim 2, wherein a pin disposed parallel tothe axis of said torsion damper assembly rotatably mounts saidintervening means of said one of said coaxial parts.
 4. The torsiondamper assembly of claim 3, wherein said intervening means comprises anose for circumferentially hooking said one resilient member in saidoperative position of said intervening means.
 5. The torsion damperassembly of claim 1, comprising an axially extending support meansassociated with said intervening means for providing saidcircumferential abutment in said operative position and an annular plateassociated with said intervening means and having recesses receivingsaid resilient members between said coaxial parts, in circumferentialengagement with said one resilient member and with circumferentialclearance with respect to the other of said resilient members, saidannular plate carrying a selected one of said intervening means and saidsupport means, the nonselected one of said intervening means and saidsupport means being part of a component fixed relative to one of saidcoaxial parts, whereby said annular plate cooperates with saidintervening means to provide circumferential support for said oneresilient member.
 6. The torsion damper assembly of claim 5, whereinsaid intervening means is received in guide means, said support meansbeing located in the path of movement of said intervening means in saidguide means.
 7. The torsion damper assembly of claim 6, wherein saidintervening means comprises a plate and said guide means defines aradial recess in a flange.
 8. A torsion damper assembly of claim 7,wherein said flange has a retaining portion protruding into said radialrecess at the radially outer end thereof beyond a shoulder cooperablefor radial abutment with said retaining nose, said intervening meanshaving a radial extension against which said support means is adapted tobear.
 9. The torsion damper assembly of claim 6 wherein said guide meansis part of said one coaxial part, said support means being carried bysaid annular plate.
 10. The torsion damper assembly of claim 6, whereinsaid return means opposing said intervening means comprises a torsionspring the middle part of which encircles a hub fixed to a first of saidcoaxial parts, one arm of said torsion spring being attached to saidintervening means.
 11. The torsion damper assembly of claim 10, whereinfor movement of said intervening means from said standby position tosaid operative position, said torsion spring arm comprises a bendcooperable with a shoulder fixed to a second of said coaxial parts, saidmiddle part of said torsion spring being fixed angularly relative tosaid hub.
 12. The torsion damper assembly of claim 10, wherein formovement of said intervening means from said standby position to saidoperative position, cam means are operatively disposed between saidintervening means and said annular plate.
 13. The torsion damperassembly of claim 12, wherein said cam means comprise finger meanscarried axially on said annular plate, and a slot having an inclinededge being formed in said intervening means, said finger means beingreceived in said slot.
 14. The torsion damper assembly of claim 13,wherein said slot has a circumferential extension.
 15. The torsiondamper assembly of claim 1, wherein at least two said intervening meansare arranged in diametrically opposite directions.
 16. The torsiondamper assembly of claim 10, wherein at least two said intervening meansare arranged in diametrically opposite positions, said return meanshaving two said arms attached to the respective intervening means, saidmiddle part of said torsion spring comprising two coiled wire portionswith a U-bend therebetween.
 17. The torsion damper assembly of claim 1,wherein said intervening means comprises a lever, one end of said leverbeing pivotally mounted on said one coaxial part about an axis parallelto the axis of said torsion damper assembly, said support meanscomprising an abutment shoulder on said annular plate, the other end ofsaid lever being cooperable with said abutment shoulder.
 18. The torsiondamper assembly of claim 17, wherein for movement of said lever fromsaid standby position to said operative position, cam means are providedbetween said lever and said annular plate.
 19. The torsion damperassembly of claim 18, wherein said lever extends substantially chordallyof said torsion damper assembly.
 20. The torsion damper assembly ofclaim 1, wherein said intervening means comprises a bolt member mountedfor sliding movement in a guide fixed relative to said one coaxial part,a ring disposed facing said guide being fixed relative to the other ofsaid coaxial parts, a transverse opening in said ring defining a keepermember for said bolt member, said ring being mounted on a hub having asurface with cam means cooperable with said bolt member for urging thelatter toward said opening in said ring.
 21. The torsion damper assemblyof claim 1, wherein friction means are provided for delaying themovement of said intervening means from said operative position to saidstandby position under the action of said return means.
 22. The torsiondamper assembly of claim 1, wherein the number of said resilient membercontrolled by said intervening means is such that a said radialcomponent is produced relative to the axis of said torsion damperassembly.
 23. The torsion damper assembly of claim 1, wherein all saidresilient members are affected by said intervening means.
 24. Thetorsion damper assembly of claim 21, wherein the number of saidresilient member controlled by said intervening means is such that asaid radial component is produced relative to the axis of said torsiondamper assembly.
 25. The torsion damper assembly of claim 21, whereinall of said resilient members are affected by said intervening means.26. The torsion damper assembly of claim 7, wherein said guide means ispart of said one coaxial part, said support means being carried by saidannular plate.
 27. The torsion damper assembly of claim 8, wherein saidguide means is part of said one coaxial part, said support means beingcarried by said annular plate.
 28. A torsion damper assembly of the typeincluding at least two coaxial parts mounted for relative angularmovement within predetermined limits of angular displacement, saidcoaxial parts comprising respective parallel annular components,circumferentially acting resilient members opposing said relativeangular movement of said coaxial parts over at least a range of angulardisplacement, each of said resilient members being housed partly in anaperture formed in the annular component of one of said coaxial partsand partly in an aperture in the annular component of the other of saidparts, said apertures having circumferentially opposed radial edges, theimprovement comprising at least one intervening means responsive torelative angular displacement between said coaxial parts for movementbetween a standby position and an operative position beyond apredetermined relative angular displacement, return means for urgingsaid intervening means from said operative position toward said standbyposition, said intervening means effectively taking the place of one ofthe radial edges of one of said apertures housing one of said resilientmembers in said operative position of said intervening means.
 29. Atorsion damper assembly according to claim 1, wherein said one of saidresilient members has a circumferential abutment which preventsdisablement thereof.
 30. A torsion damper assembly according to claim29, wherein there is at least one relatively weak resilient member, saidintervening means providing circumferential engagement for saidrelatively stiff resilient member so that the relatively weak resilientmember does not act alone at low values of torque on pull backoperation.